Removal of metallic coatings as part of refurbishment efforts for articles removed from service requires time consuming operations. A current method for removal of metallic coatings requires smut removal after an initial grit blast for articles removed from turbine service. Smut is formed by an initial acid bath immersion, the acid bath interacting with the metallic coating. The smut formed is tightly adherent. The tightly adherent smut is then removed by at least two additional cycles of grit blasting, masking of cooling holes to prevent acid from entering the cooling holes during subsequent dip and re-dipping until the smut is removed.
The current method requires at least three grit blasting operations per stripping cycle and at least two acid stripping cycles, the acid stripping being accomplished by very strong acids, such as 20-40 weight percent nitric acid. Masking of sensitive portions of the substrate may be necessary to prevent damage either by overexposure to the chemicals or to the grit blasting operations. The additional grit blasting to remove the smut formed by the acid dip increases the risk of over-blasting, which could undesirably remove substrate material from the article and possibly removing additional substrate material during subsequent acid dips. Some methods may utilize an elevated caustic treatment at temperatures of 140° F. (60° C.) or higher. In addition to the possibility of damaging the substrate materials, the high concentrations of acid, the multiple grit blasting operations and, when used, the high temperatures of the caustic treatments create environmental, health and safety (EHS) concerns as a result of the chemicals and the multiple frit blasting operations.
What is needed is method that can remove the smut formed by exposure to an acid bath during refurbishment of a coated article removed from service, such as a coated turbine component, which reduces the possibility of damage to the substrate while minimizing exposure of the article and personnel to chemicals that may create EHS concerns.